An inductively coupled plasma (ICP) is formed by coupling the energy from a radio-frequency (typically 2 kW at 27-50 MHz) magnetic field to free electrons in a suitable gas. The magnetic field may be produced by a two-or three-turn water-cooled coil and the electrons are accelerated around the magnetic field lines that run axially through the coil. The plasma must be struck by introducing "seed" electrons into the gas, for example by means of a spark discharge, but once these are present in sufficient quantity and have enough energy the plasma becomes self-sustaining. The power from the radio frequency field is coupled directly into the plasma which can reach 10,000.degree. in its hottest regions. If a sample is introduced into the plasma it is atomized and may be analyzed, typically for elemental composition, by atomic emission spectroscopy or by mass spectrometry. In the former case, radiation from the plasma is spectroscopically analyzed while in the latter case ions generated in the plasma are sampled from the plasma and introduced into a mass analyzer.
In prior ICP torch power supplies the plasma induction coil is connected in a tuned circuit which is energized by an RF power generator at its resonant frequency. In this way optimum efficiency is obtained because the reflected power from the coil is minimum at the resonant frequency. However, the impedance of the coil, and hence the resonant frequency of the tuned circuit, vary with the state of the plasma. For example, there are significant changes in the resonant frequency when the plasma is ignited and when the composition of the sample and gas flows to the plasma are changed. Two solutions to this problem are known. First, a fixed-frequency. RF generator may be used in conjunction with a tuned circuit and matching network comprising variable capacitors and for inductors which are motor driven in a servo-control loop arranged to maintain the resonant frequency of the coil tuned circuit at the generator frequency. Such a system is described in U.S. Pat. No. 4,629,940. Although extensively used it requires the provision of physically large and expensive variable capacitors and motors and because of the relatively slow speed at which the motors can operate, is not particularly effective at coping with sudden changes in the state of the plasma.
The second prior method, described in several variations in U.S. Pat. Nos. 3,958,883 and 4,337,415, European Patent 281157, and by H. Linn in ICP Information Newsletter 1976 vol 2 (2), employs what is known as a "free-running" oscillator in which the tuned circuit comprising the plasma induction coil is also the circuit element which determines the frequency of oscillation. Free-running oscillators automatically achieve optimum power transfer but suffer several other disadvantages in practice, for example, the major power components (e.g. thermionic tube, etc) of the RF generator must be mounted close to the plasma induction coil, resulting in a physically large torch assembly which is inconvenient in use. Free-running oscillators used for ICP torches are also more difficult to start than a fixed-frequency oscillator and are difficult to implement with solid-state electronics.
A variation of the free-running oscillator ICP torch power supply is disclosed in U.S. Pat. No. 4,629,887. In this generator, the frequency of the oscillator is determined by a parallel resonant circuit which is capacitively coupled to the plasma induction coil. The coupling capacitor and induction coil are arranged to present a substantially resistive load to the generator under normal plasma conditions. This arrangement provides improved operating stability and exhibits less frequency variation with changing plasma conditions than the conventional free-running oscillator.
It is an object of the invention to provide an ICP emission spectrometer or mass spectrometer having an improved torch RF power generator which overcomes or substantially mitigates the disadvantages of prior generators, and which can easily be implemented with solid state power devices. It is a further object to provide means by which optimum power transfer from the RF generator to the plasma is automatically maintained, irrespective of the state of the plasma.